Coated sodium percarbonate granules with improved storage stability

ABSTRACT

The invention concerns coated sodium percarbonate granules comprising a core produced by fluidised bed spray granulation, containing sodium percarbonate as its main component, an inner coating layer containing an organic, hydrate-forming salt as its main component, and an outer coating layer containing as its main component at least one surfactant containing in the molecule one or more sulfate or sulfonate groups in the form of an alkali metal, alkaline-earth metal or ammonium salt. The invention also concerns a process for the production of the granules, the use of the granules in bleaching and cleaning agents, and bleaching and cleaning agents containing the granules. The sodium percarbonate granules according to the invention display improved storage stability combined with high internal stability and a high active oxygen content.

The invention concerns coated sodium percarbonate granules with improved storage stability and a process for the production of these sodium percarbonate granules.

Sodium percarbonate is used as a bleaching agent and as an active bleaching component in detergents and cleaning agents. The disadvantage of sodium percarbonate here is that it tends to decompose in contact with moisture and with components of the detergent and cleaning agent compositions, leading to a loss in active oxygen and hence in bleaching action. In these applications sodium percarbonate is therefore preferably used in the form of granules coated with a stabilising coating layer in order to achieve improved storage stability in detergents and cleaning agents.

It is known from EP-A 0 863 842 that sodium percarbonate granules with improved storage stability can be produced by applying a firmly adhering coating layer, consisting substantially of sodium sulfate, to a core of sodium percarbonate produced by fluidised bed spray granulation.

WO 96/06801 describes coated sodium percarbonate granules having improved stability that are synthesised from a core of sodium percarbonate granules, an inner coating layer containing a sulfate or silicate and an outer coating layer containing a compound (B) selected from salts of monocarboxylic and dicarboxylic acids having 4 or more carbon atoms and a compound (C) selected from salts of sulfated oils, α-olefin sulfonic acids and monoalkyl and dialkyl sulfosuccinates. The examples given in the specification show that the presence of compounds (B) is necessary to achieve an adequate storage stability and that granules containing only compound (C) and no compound (B) in the outer layer display unsatisfactory storage stability.

JP-A 06-263434 describes stable in storage sodium percarbonate granules that are synthesised from surfactant-containing sodium percarbonate granules as the core, an inner coating layer consisting of a sulfate and an outer coating layer consisting of an anionic surfactant. Here the core material is produced by crystallisation and subsequent granulation with addition of the surfactant. From the examples and comparative examples given in the specification it follows that the core material must be produced with addition of a surfactant in order for the coated sodium percarbonate obtained to display an adequate storage stability and that granules with a surfactant-free sodium percarbonate core, an inner coating layer consisting of a sulfate and an outer coating layer consisting of an anionic surfactant display insufficient storage stability.

In the case of the sodium percarbonate granules described in EP-A 0 863 842 there is still a need for an improvement in storage stability. The sodium percarbonate granules described in WO 96/06801 have the disadvantage that they are complicated to produce, since at least three different compounds are necessary for coating the sodium percarbonate granules.

The sodium percarbonate granules described in JP-A 06-263434 have the disadvantage that with the surfactant they contain an organic compound in the sodium percarbonate core. This has a disadvantageous effect on the internal stability of the sodium percarbonate granules and leads to restrictions in the storage of large amounts of the granules in silos. For storage in silos the sodium percarbonate granules must display high internal stability, in other words low heat release during storage, since otherwise decomposition of the sodium percarbonate granules can occur in the silo due to self-accelerating heating.

The object of the invention was therefore to provide coated sodium percarbonate granules having improved storage stability, which can be produced by simple means and which combine the properties of high storage stability in detergent and cleaning agent compositions and high internal stability for storage in silos.

Surprisingly it was found that this object can be achieved by coated sodium percarbonate granules comprising

-   a) a core produced by fluidised bed spray granulation, containing     sodium percarbonate as its main component, -   b) an inner coating layer containing an inorganic, hydrate-forming     salt as its main component and -   c) an outer coating layer containing as its main component at least     one surfactant containing in the molecule one or more sulfate or     sulfonate groups in the form of an alkali metal, alkaline-earth     metal or ammonium salt.

The invention also includes a process for the production of coated sodium percarbonate granules with improved storage stability, comprising the following steps:

-   a) production of a core material by fluidised bed spray granulation,     wherein an aqueous hydrogen peroxide solution and an aqueous soda     solution are sprayed into a fluidised bed comprising sodium     percarbonate particles, -   b) application of an inner coating layer onto the core material from     step a) by spraying an aqueous solution in which at least one     hydrate-forming inorganic salt is dissolved and -   c) application of an outer coating layer onto the coated material     from step b) by spraying an aqueous mixture in which at least one     surfactant is dissolved or emulsified, which contains in the     molecule one or more sulfate or sulfonate groups in the form of an     alkali metal, alkaline-earth metal or ammonium salt.

The invention furthermore relates to the use of the sodium percarbonate granules according to the invention as bleaching agents or as active bleaching components in detergent and cleaning agents, and to bleaching agents and detergents and cleaning agents containing the sodium percarbonate granules according to the invention.

The sodium percarbonate granules according to the invention comprise a core produced by fluidised bed spray granulation, containing sodium percarbonate as its main component. Production of the core material takes place by known means by fluidised bed spray granulation, wherein an aqueous hydrogen peroxide solution and an aqueous sodium carbonate solution are sprayed into a fluidised bed containing nuclei whose dimensions are smaller than the particles of the core material to be produced and wherein water is evaporated at the same time. Sodium percarbonate is preferably used as the nucleus material. The core material is preferably produced by the method described in EP-A 0 716 640.

The proportion of core material in the coated sodium percarbonate granules according to the invention is preferably in the range from 80 to 99.5 wt. %, particularly preferably in the range from 90 to 99 wt. % and in particular in the range from 92 to 98 wt. %. The proportion of sodium percarbonate in the core of the sodium percarbonate granules according to the invention is preferably more than 80 wt. %, particularly preferably more than 95 wt. % and in particular more than 98 wt. %. The proportion of organic carbon compounds in the core is preferably less than 1 wt. %, particularly preferably less than 0.1 wt. % and in particular less than 0.01 wt. %.

In a preferred embodiment the core contains small amounts of additives, which have a stabilising effect on the active oxygen content, the proportion of stabilising additives in the core being preferably less than 2 wt. %. Magnesium salts, water glass, stannates, pyrophosphates, polyphosphates and chelating agents from the series comprising hydroxycarboxylic acids, aminocarboxylic acids, aminophosphonic acids, phosphonocarboxylic acids and hydroxyphosphonic acids, as well as alkali metal, ammonium or magnesium salts thereof, are preferably used as stability-increasing additives. In a particularly preferred embodiment the core contains as stabilising additive an alkali metal silicate, preferably water glass, having a SiO₂/Na₂O modulus in the range from 1 to 3, in a quantity from 0.1 to 1 wt. %. In the most preferred embodiment the core also contains in addition to this amount of alkali metal silicate a magnesium compound in a quantity of 50 to 2000 ppm Mg²⁺.

The coated sodium percarbonate granules according to the invention also include in addition to the core of sodium percarbonate an inner coating layer containing an inorganic, hydrate-forming salt as its main component and an outer coating layer containing as its main component at least one surfactant displaying in the molecule one or more sulfate or sulfonate groups in the form of an alkali metal, alkaline-earth metal or ammonium salt. In addition to this inner and outer coating layer, the sodium percarbonate granules according to the invention can also contain one or more additional coating layers, these being able to be positioned both between the core and the inner coating layer and between the inner and the outer coating layer and outside the outer coating layer.

Between the coating layers and between the innermost coating layer and the core there can be a clear boundary, at which the composition changes abruptly. Generally, however, a transition zone will form between the individual coating layers and between the innermost coating layer and the core, which contains the components from both adjoining layers. Such transition zones are formed for example by the application of a coating layer in the form of an aqueous solution, whereby as the layer starts to form, part of the layer beneath it is partially dissolved, such that a transition zone forms that contains the components of both layers. In this way, between the core and the inner coating layer a transition layer can form which contains sodium percarbonate, sodium carbonate, sodium hydrogen carbonate and the inorganic hydrate-forming salt from the inner coating layer. Similarly, between the inner coating layer and the outer coating layer a transition layer can form which contains the inorganic hydrate-forming salt from the inner coating layer and the surfactant from the outer coating layer.

The inner coating layer and outer coating layer are preferably formed in such a way that they cover the underlying material by more than 95%, preferably more than 98% and in particular completely.

The inner coating layer of the coated sodium percarbonate granules according to the invention contains an inorganic, hydrate-forming salt as its main component. The proportion of inner coating layer in the coated sodium percarbonate granules according to the invention is preferably in the range from 1 to 10 wt. %, particularly preferably in the range from 2 to 7 wt. %. The proportion of inorganic, hydrate-forming salt in the material of the inner coating layer is preferably at least 50 wt. %, particularly preferably at least 90 wt. %. The inorganic, hydrate-forming salt of the inner coating layer is preferably chosen from the series comprising sodium sulfate, sodium carbonate, sodium hydrogen carbonate or magnesium sulfate. Mixtures and mixed salts of these compounds are also suitable. The inner coating layer particularly preferably contains sodium sulfate as the inorganic hydrate-forming salt.

The outer coating layer of the coated sodium percarbonate granules according to the invention contains as its main component at least one surfactant containing in the molecule one or more sulfate or sulfonate groups in the form of an alkali metal, alkaline-earth metal or ammonium salt. The proportion of outer coating layer in the coated sodium percarbonate granules according to the invention is preferably in the range from 0.1 to 10 wt. %, particularly preferably 0.5 to 5 wt. % and in particular 1 to 3 wt. %. The proportion of surfactant in the material of the outer coating layer is preferably at least 50 wt. % and particularly preferably at least 80 wt. %. In a preferred embodiment the outer coating layer contains a mixture of surfactants, the surfactant mixture consisting of more than 50 wt. % of surfactants that contain in the molecule one or more sulfate or sulfonate groups in the form of an alkali metal, alkaline-earth metal or ammonium salt. The sulfate or sulfonate groups in the surfactant in the outer coating layer are particularly preferably in the form of an alkali metal salt, most preferably in the form of the sodium salt.

In the outer coating layer alkyl benzene sulfonates, alkane sulfonates, α-olefin sulfonates, α-sulfo fatty acid esters or sulfosuccinates are preferably used as surfactants having sulfonate groups. As alkyl benzene sulfonates, preferably those a straight-chain or branched alkyl group with preferably 8 to 20 carbon atoms, particularly 10 to 16 carbon atoms, are used. Preferred alkane sulfonates are examples having straight-chain alkyl chains with 12 to 18 carbon atoms. As α-olefin sulfonates, the reaction products from the sulfonation of α-olefins having 12 to 18 carbon atoms are preferably used. As α-sulfo fatty acid esters, sulfonation products of fatty acid esters produced from fatty acids having 12 to 18 carbon atoms and short-chain alcohols having 1 to 3 carbon atoms are preferred.

Alkyl sulfates and ether sulfates are preferably used as surfactants having a sulfate group in the molecule. Preferred alkyl sulfates are examples having straight-chain alkyl radicals with 12 to 18 carbon atoms. Preferred ether sulfates are the alkyl ether sulfates obtained by ethoxylation of linear alcohols having 12 to 18 carbon atoms with 2 to 6 ethylene oxide units, followed by sulfation.

In addition to the surfactants having sulfate or sulfonate groups, the outer coating layer can also contain other anionic surfactants as well as non-ionic surfactants, cationic surfactants and amphoteric surfactants.

In a further embodiment of the invention, corresponding to DE 102 61 161, the coated sodium percarbonate granules display an additional coating layer on top of the inner coating layer which contains as its main component an alkali metal silicate having a modulus of SiO₂ to alkali metal oxide of over 2.5. The additional coating layer can either be between the inner and the outer coating layer or be positioned on top of the outer coating layer. The proportion of additional coating layer in the coated sodium percarbonate granules according to the invention is preferably in the range from 0.2 to 3 wt. %. The proportion of alkali metal silicate in the material of the additional coating layer is preferably more than 50 wt. % and particularly preferably more than 80 wt. %. Sodium water glass is preferably used as the alkali metal silicate in the additional coating layer.

The sodium percarbonate granules according to the invention preferably display an average particle diameter in the range from 0.3 to 2 mm, particularly preferably in the range from 0.5 to 1 mm. The sodium percarbonate granules according to the invention preferably contain only a small proportion of fine particles. More than 90% of the particles in the sodium percarbonate according to the invention preferably display a diameter of over 0.2 mm.

The invention also concerns a process for the production of coated sodium percarbonate granules with improved storage stability, which comprises the following steps:

-   a) production of a core material by fluidised bed spray granulation,     wherein an aqueous hydrogen peroxide solution and an aqueous soda     solution are sprayed into a fluidised bed comprising sodium     percarbonate particles, -   b) application of an inner coating layer onto the core material from     step a) by spraying an aqueous solution, in which at least one     hydrate-forming, inorganic salt is dissolved and -   c) application of an outer coating layer onto the coated material     from step b) by spraying an aqueous mixture in which at least one     surfactant is dissolved or emulsified, which contains in the     molecule one or more sulfate or sulfonate groups in the form of an     alkali metal, alkaline-earth metal or ammonium salt.

Production of the core material from hydrogen peroxide and soda takes place by known means by the fluidised bed spray granulation method and preferably by the method described in EP-A 0 716 640. Here hydrogen peroxide and sodium carbonate solution are preferably reacted in a molar ratio of H₂O₂ to Na₂CO₃ of 1.4 to 1.7, particularly preferably 1.5 to 1.65. Hydrogen peroxide is used as an aqueous solution with preferably 30 to 75 wt. % H₂O₂, particularly preferably 40 to 70 wt. % H₂O₂. The hydrogen peroxide solution can additionally contain stabilising additives, such as e.g. complexing agents or magnesium compounds. Sodium carbonate is preferably used as an aqueous solution with a concentration of between 10 wt. % sodium carbonate and the saturation concentration of sodium carbonate, particularly preferably between 20 wt. % sodium carbonate and the saturation concentration of sodium carbonate. The sodium carbonate solution can likewise contain stabilising additives, such as e.g. water glass. In the fluidised bed spray granulation process the water introduced with the feed materials is evaporated and removed by feeding a drying gas into the fluidised bed. Air or a combustion gas obtained by burning a fuel such as e.g. natural gas with air is preferably used as the drying gas. The drying gas is preferably supplied to the fluidised bed at a temperature of between 120 and 400° C., particularly preferably between 200 and 400° C. The temperature in the fluidised bed is preferably kept at between 40 and 95° C., particularly between 40 and 80° C. and in particular between 50 and 70° C.

In a preferred embodiment, nucleus material is supplied to the fluidised bed in a quantity that leads to the formation of granules having an average particle size in the range from 0.2 to 2 mm. The core material is preferably discharged from the fluidised bed by a classifying process, and particularly preferably by the process described in EP-A 0 938 922, such that preferably more than 90 wt. % of the core material particles discharged from the fluidised bed display a diameter of over 0.2 mm.

The inner coating layer is applied by spraying an aqueous solution in which at least one hydrate-forming, inorganic salt is dissolved. The inner coating layer is preferably applied by spraying an aqueous sodium sulfate solution. The bulk of the water contained within the aqueous solution is preferably evaporated by the input of heat as it is being sprayed, such that during the application of the inner coating layer only a small part of the core material begins to dissolve again and a solid coating layer is already formed from the hydrate-forming, inorganic salt during the spraying process. The inner coating layer is preferably applied by spraying the aqueous solution of the hydrate-forming, inorganic salt in a fluidised bed and particularly preferably by the process described in EP-A 0 970 917, with which a dense coating layer can be achieved even with small amounts of coating layer material. Application of the inner coating layer in a fluidised bed preferably takes place with the supply of a drying gas to the fluidised bed, such that a temperature in the range from 30 to 90° C. is established in the fluidised bed.

Application of the outer coating layer by the process according to the invention is performed by spraying an aqueous mixture containing at least one surfactant which contains in the molecule one or more sulfate or sulfonate groups in the form of an alkali metal, alkaline-earth or ammonium salt, onto the granules coated with an inner coating layer. The aforementioned alkyl benzene sulfonates, alkyl sulfonates, α-olefin sulfonates, sulfosuccinates, alkyl sulfates and ether sulfates are preferably used as surfactants, either alone or in blends. The surfactants or surfactant blends can be sprayed both as aqueous solutions and as aqueous emulsions. Solid surfactants that are poorly soluble in water can be converted into sprayable aqueous emulsions by heating them above their melting point, by adding an auxiliary surfactant, or by a combination of these two measures, in the presence of water. During the application of the outer coating layer, the bulk of the water contained within the aqueous surfactant blend is preferably evaporated by the input of heat as the aqueous blend is being sprayed, such that during the application of the outer coating layer only a small part of the inner coating layer begins to dissolve again and a solid or waxy, surfactant-containing outer coating layer is formed as the outer coating layer is applied. The outer coating layer is preferably applied by spraying the aqueous surfactant blend in a fluidised bed in which the material coated with the inner coating layer has been placed. The outer coating layer is particularly preferably applied by the process described in EP-A 0 970 917, with which dense outer coating layers can be achieved even with small amounts of surfactant. During application of the outer coating layer by spraying the aqueous surfactant blend in a fluidised bed, a drying gas is preferably supplied to the fluidised bed such that a temperature in the range from 30 to 90° C. is established in the fluidised bed.

In a preferred embodiment corresponding to WO 02/051745, the process according to the invention for producing coated sodium percarbonate granules includes an additional step of thermal aftertreatment at a temperature of 70 to 120° C., preferably 80 to 95° C., for a period of at least 2 minutes, preferably 5 to 60 minutes, which is performed after step a), production of the core material by fluidised bed spray granulation. The thermal aftertreatment is preferably performed after step b), application of an inner coating layer, and particularly preferably after step c), application of the outer coating layer.

The invention also additionally provides the use of the coated sodium percarbonate granules according to the invention, or of the coated sodium percarbonate granules produced by the process according to the invention, as bleaching agents and as active bleaching components in detergents and cleaning agents.

The invention also provides bleaching agents having improved storage stability, which contain 15 to 99 wt. % of the coated sodium percarbonate granules according to the invention or of the sodium percarbonate granules produced by the process according to the invention.

The invention likewise provides detergents and cleaning agents having improved storage stability, which contain 1 to 40 wt. % of the coated sodium percarbonate granules according to the invention or of the coated sodium percarbonate granules produced by the process according to the invention.

The coated sodium percarbonate granules according to the invention and the coated sodium percarbonate granules produced by the process according to the invention display improved storage stability in detergents and cleaning agents. In commercial, zeolite-containing detergent blends, which are stored in commercial detergent packs, the coated sodium percarbonate granules according to the invention display an active oxygen loss of less than 20% when stored at 35° C. and 80% relative humidity for a period of 8 weeks, in other words the residual active oxygen after storage for eight weeks is over 80% of the original amount of active oxygen.

The coated sodium percarbonate granules according to the invention and the coated sodium percarbonate granules produced by the process according to the invention also display a high internal stability, i.e. when stored in bulk they display only a low release of heat through decomposition reaction. After storage for 48 hours at 40° C. the amount of heat released by the coated sodium percarbonate granules according to the invention, measured by the TAM method, is less than 5 μW/g and preferably less than 3 μW/g. Determination of the heat release by the TAM method is performed by microcalorimetry using a thermal activity monitor supplied by Thermometric AB, Järfälla (SE) over a 48 hour storage period in a measuring cell at 40° C.

High storage stability combined with high internal stability can be achieved in the coated sodium percarbonate granules according to the invention even with small quantities of coating agents and additives. The coated sodium percarbonate granules according to the invention thus display a high content of sodium percarbonate and as a consequence a high active oxygen content. The active oxygen content, determined by redox titration of the hydrogen peroxide released after dissolving the granules in water, is preferably more than 13.0 wt. % and is particularly preferably at least 13.5 wt. %.

Coated sodium percarbonate granules according to the invention displaying no other coating layers in addition to the inner and outer coating layer, exhibit short dissolving times when the granules are dissolved in water. For these sodium percarbonate granules coated according to the invention the times to dissolve 95% of the granules in water at 15° C., determined by conductometry, are less than 50% longer than the times to dissolve non-coated sodium percarbonate core materials.

By contrast, coated sodium percarbonate granules according to the invention displaying an additional coating layer which contains as its main component an alkali metal silicate having a modulus of over 2.5, exhibit a delayed dissolution in water, with dissolving times of preferably more than 5 minutes, particularly preferably more than 10 minutes. The granules having delayed dissolution display advantages when used in detergents and cleaning agents containing oxidation-sensitive enzymes. Through the delayed dissolution of the granules, the enzymes can act over a longer period of time before they are deactivated by the released active oxygen use.

EXAMPLES

-   a) Determining the dissolving time:     -   2 g of non-coated or coated sodium percarbonate are stirred into         1 l water at 15° C. The dissolving time is defined as the time         at which 95% is dissolved; this is determined by conductometry.     -   To compare the dissolving time of two differently         produced/coated products, they must display a comparable grain         size; if necessary, narrow screen fractions are compared with         one another. -   b) Determining the storage stability in detergent blends:     -   A phosphate-free but zeolite-containing detergent powder, TAED         activator and the coated sodium percarbonate are mixed together         in a quantity such that the mixture contains 5% TAED and the         active oxygen content is around 2.35 wt. %.

Components of the detergent powder in wt. %: Anionic surfactants 12 Non-ionic surfactants 8 Zeolite A 36 Soda 10 Sodium silicates 3 Remainder including moisture 31

-   -   800 g of each mixture are stored in commercial E1 detergent         packs, impregnated to be water-repellent and sealed, in a         conditioning cabinet at 35° C. and 80% relative humidity. The         active oxygen content is determined in the conventional way by         permanganometry. The retained active oxygen content is         determined as a percentage from the initial active oxygen         content and the active oxygen content after 8 weeks.

Comparative Example

Using the process described in EP-B 0 716 640, sodium percarbonate granules, having an average particle diameter of 0.75 mm and a proportion of fines under 0.2 mm of less than 2%, were produced from an aqueous hydrogen peroxide solution and an aqueous soda solution by fluidised bed spray granulation at a fluidised bed temperature of 50 to 70° C.

The granules were coated with a sodium sulfate layer by the process described in EP-B 0 863 842 in section [0021] by spraying a 20 wt. % sodium sulfate solution in a fluidised bed at a fluidised bed temperature of 50 to 70° C. The amount of sodium sulfate solution was chosen such that 6 wt. % sodium sulfate, relative to the amount of granules used, was applied.

The sodium percarbonate granules coated with sodium sulfate displayed an active oxygen content of 13.7 wt. %. In the TAM measurement the granules displayed a release of heat of 2.2 μW/g after 48 hours at 40° C. The dissolving time of the granules at 15° C. in water was determined as 1.5 minutes.

After storage in a detergent blend for eight weeks at 35° C. and 80% relative humidity the residual content of active oxygen was 67% of the original amount.

Example

1000 g of the sodium sulfate-coated sodium percarbonate granules produced in the comparative example were placed in a Strea-1 laboratory coater supplied by Aeromatic and held in a fluidised bed with fluidising air at 70° C. A 2 wt. % aqueous solution of sodium dodecyl benzene sulfonate (Lutensit A-LBN from BASF) was sprayed through a two-component nozzle into the fluidised bed. The amount of surfactant solution was chosen such that 2 wt. % surfactant, relative to the amount of granules used, was applied. Following spraying of the surfactant solution, the granules were dried in the fluidised bed for a further 30 minutes at 70° C.

The sodium percarbonate granules coated according to the invention that were obtained had an active oxygen content of 13.6 wt. %. In the TAM measurement the granules displayed a release of heat of 2.1 μW/g after 48 hours at 40° C. The dissolving time of the granules at 15° C. in water was determined as 1.5 minutes.

After storage in a detergent blend for eight weeks at 35° C. and 80% relative humidity, the residual content of active oxygen was 88% of the original amount. 

1-26. (canceled)
 27. A coated sodium percarbonate with improved storage stability, consisting of granules, said granules comprising: a) a core produced by fluidised bed spray granulation, comprising sodium percarbonate as its main component; b) an inner coating layer comprising an inorganic hydrate-forming salt as its main component; and c) an outer coating layer comprising as its main component at least one surfactant, wherein each molecule of said surfactant comprises one or more sulfate or sulfonate groups in the form of an alkali metal, alkaline-earth metal or ammonium salt.
 28. The sodium percarbonate of claim 27, wherein said core makes up 80-99.5% of the weight of said granules.
 29. The sodium percarbonate of claim 27, wherein said sodium percarbonate makes up more than 80% of the weight of said core.
 30. The sodium percarbonate of claim 27, wherein said core further comprises less than 1 wt % of organic carbon compounds.
 31. The sodium percarbonate of claim 27, wherein said core further comprises 0.1-1 wt % of an alkali metal silicate.
 32. The sodium percarbonate of claim 31, wherein said core further comprises 50-2000 ppm Mg²⁺.
 33. The sodium percarbonate of claim 27, wherein said inner coating layer makes up 1-10% of the weight of said granules.
 34. The sodium percarbonate of claim 27, wherein said inorganic hydrate-forming salt makes up at least 50% of the weight of said inner coating layer.
 35. The sodium percarbonate of claim 27, wherein said inorganic hydrate-forming salt in said inner coating layer is selected from the group consisting of: sodium sulfate; sodium carbonate; sodium hydrogen carbonate; magnesium sulfate; and mixtures or mixed salts of these compounds.
 36. The sodium percarbonate of claim 35, wherein said inorganic hydrate-forming salt is sodium sulfate.
 37. The sodium percarbonate of claim 27, wherein said outer coating layer makes up 0.1-10% of the weight of said granules.
 38. The sodium percarbonate of claim 27, wherein surfactants make up at least 50% of the weight of said outer coating layer.
 39. The sodium percarbonate of claim 38, wherein said outer coating layer comprises a blend of surfactants and wherein at least 50 wt % of said surfactants in said blend comprise one or more sulfate or sulfonate groups in the form of an alkali metal, alkaline-earth metal or ammonium salt.
 40. The sodium percarbonate of claim 27, wherein said sulfate or sulfonate groups in said surfactant are in the form of a sodium salt.
 41. The sodium percarbonate of claim 27, further comprising an additional coating layer, wherein said additional coating layer covers said inner coating layer and comprises, as its main component, an alkali metal silicate having a modulus of SiO₂ to alkali metal oxide of over 2.5.
 42. The sodium percarbonate of claim 41, wherein said additional coating layer makes up 0.2-3% of the weight of said granules.
 43. The sodium percarbonate of claim 27, wherein said granules have an average particle size d₅₀ in the range of 0.3-2 mm.
 44. The sodium percarbonate of claim 27, wherein more than 90 wt. % of said granules have a diameter of more than 0.2 mm.
 45. A process for the production of coated sodium percarbonate granules with improved storage stability, comprising: a) producing a core by fluidised bed spray granulation, wherein an aqueous hydrogen peroxide solution and an aqueous soda solution are sprayed into a fluidised bed comprising sodium percarbonate particles; b) applying an inner coating layer onto the core of step a) by spraying an aqueous solution in which at least one hydrate-forming inorganic salt is dissolved; and c) applying an outer coating layer onto the coated core of step b) by spraying an aqueous mixture in which at least one surfactant is dissolved or emulsified and wherein each molecule of said surfactant comprises one or more sulfate or sulfonate groups in the form of an alkali metal, alkaline-earth metal or ammonium salt.
 46. The process of claim 45, wherein said hydrate-forming inorganic salt in step b) is sodium sulfate.
 47. The process of claim 45, wherein said inner coating layer and said outer coating layer are applied by spraying in a fluidised bed.
 48. The process of claim 47, wherein said inner coating layer and said outer coating layer are applied at a temperature of said fluidised bed in the range from 30 to 90° C.
 49. A composition for bleaching or cleaning, wherein said composition comprises a coated sodium percarbonate according to claim
 27. 50. The composition of claim 49, wherein said composition is a bleaching agent and said coated sodium percarbonate makes up 15-99% of the weight of said composition.
 51. The composition of claim 49, wherein said composition is a detergent or cleaning agent and said sodium percarbonate makes up 1-40% of the weight of said composition. 